Steam generators of pressurized water nuclear reactors comprise a bundle of tubes, the ends of which are swaged in a very thick tube plate. The primary water of the reactor, at high pressure and at high temperature, which comes into contact with the fuel assemblies of the core, circulates within the tubes of the bundle so as to heat and vaporize the supply water, brought into contact with the outer wall of the tubes, inside the casing of the steam generator.
The tubes of the bundle are flush with one of the faces of the plate, called the inlet face, via which the primary water enters the bundle and leaves it again in two separate zones, and emerge from this tube plate so as to penetrate into the casing of the bundle via its other, outlet face.
The wall of the tubes of the bundle therefore constitutes a barrier between the primary fluid containing radioactive products and the supply water to be vaporized, the steam of which is conveyed to the turbine associated with the nuclear reactor. It is therefore necessary as far as possible to prevent a leak from occurring in the tubes of the steam-generator bundle and to repair the tubes efficiently and as quickly as possible when their wall is perforated.
Because of the temperature gradients, mechanical stresses attributed to the pressure differences and to the different forms of corrosion can arise in the steam generator, both on the primary side and on the secondary side, during the use of the steam generator, the service life of which must be equal to that of the other parts of the reactor, and tube perforations which produce leaks can occur and require repair during reactor maintenance periods.
To date, one of the techniques used most often has been to render the defective tube inoperative by means of a plug fastened to the end of the tube adjacent to the inlet face of the tube plate, in its part receiving the primary water coming from the reactor core. This known technique has a reasonably high degree of reliability, but a disadvantage is that the exchange surface of the steam generator is reduced, all the more since a larger number of tubes of the bundle will have had to be put out of action.
Processes for repairing the tubes of the bundle of a steam generator by lining these tubes at the location of the defect causing a loss of leak-proofing capacity have therefore been conceived. In these, a tubular lining having an outside diameter a little less than the inside diameter of the tube to be repaired and sufficient length to cover the defect is introduced into the tube via the inlet face of the tube plate and is placed on this tube plate and in the tube so as to be flush with the inlet face. This lining is subsequently secured by diametrical expansion inside the tube.
It has been proposed, for example, to carry out this expansion by means of a hydraulic mandrel, but the mechanical strength and leak-proofing of the repaired tube have proved inadequate.
It has also been proposed to improve the fastening of the lining by swaging it inside the tube. Swaging results in a rolling of the lining within the tube and therefore in a reduction in its thickness. However, such a swaging operation to fasten the lining with sufficient strength is only possible in the part of the tube located inside the tube plate.
It has therefore been proposed to weld the lining to the inside of the tube after it has been expanded hydraulically. However, the welding temperatures with or without a filler metal make it necessary to operate in a protected environment, and this complicates the repair operations on the tubes. Consequently, it is usually preferable to fasten the lining in the tube by brazing, after the former has been deformed by hydraulic pressure.
The end of the lining flush with the inlet face of the tube plate is therefore fastened inside the tube by swaging within the tube plate, and the connection is made leak-proof by a weld at the end of the tube and the lining.
The lining is then explained radially inside the tube by hydraulic means in a second zone of this lining outside the tube plate. Fastening is completed by fusion of a brazing material previously introduced between the outer surface of the lining and the inner surface of the tube in the zone undergoing hydraulic expansion.
Such a process makes it possible to ensure satisfactory mechanical fastening of the lining within the tube, but it has been noticed that the tube and the lining undergoes increased corrosion above the zone of the junction made by means of hydraulic expansion and brazing. This increased corrosion can be linked to the presence of a gap between the tube and the lining in the end part of the tube opposite its end swaged in the tube plate. In fact, it is not possible to carry out hydraluic expansion up to the end part of the tube, since it is necessary, in this part, to ensure satisfactory leak-proofing of the expansion chamber of the inner wall of the lining.
When a mandrel with a deformable wall is used, it is also necessary to place this mandrel at a certain distance from the upper end of the lining inside the tube.
In all cases, therefore, there is a dead zone of a certain length, in which radial expansion of the lining within the tube does not take place. This results in an annular gap between the lining and the tube, within which corrosion products or radioactive products are deposited, and these impair the operating conditions and lower the corrosion resistance of the tube.
The importance of this dead zone in the end part of the tube had been underestimated up to then, and the main aim had been to make a mechanically strong connection between the lining and the tube.